What causes a sausage digit on radiography, ultrasound and MRI?
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Answer:
A sausage digit, or dactylitis, is uniform fusiform swelling of an entire finger or toe, so that the whole digit rather than a single joint is enlarged and the normal contour between joints is lost. It is a hallmark of the spondyloarthritides, above all psoriatic arthritis, and also occurs in reactive arthritis, enteropathic arthritis and ankylosing spondylitis. Radiographs show soft-tissue swelling of the whole digit and, later, the erosive and proliferative changes of psoriatic arthritis. Ultrasound and MRI show that the dominant lesion is flexor tenosynovitis, accompanied by diffuse soft-tissue oedema and variable synovitis; extracapsular soft-tissue oedema predominates in early dactylitis while synovitis becomes more prominent in chronic disease. Important non-spondyloarthritic causes must be excluded: sickle cell dactylitis (hand-foot syndrome) in an infant, tuberculous dactylitis (spina ventosa) and non-tuberculous mycobacterial infection, sarcoidosis, and bacterial flexor sheath infection, which is a surgical emergency.
Why is it called so?
The uniformly swollen digit takes on the plump, fusiform shape of a sausage.
Pathophysiology
In psoriatic arthritis the primary lesion is enthesitis. Inflammation at the entheses of the flexor tendon sheath, the collateral ligaments and the joint capsules of the digit spreads beyond the capsule into the surrounding soft tissue, so oedema is not confined to a joint but involves the whole digital ray. Flexor tenosynovitis contributes most of the volume, because the sheath extends the length of the digit, and diffuse extracapsular oedema fills the remainder. The distribution of inflammation along the whole tendon and its entheses rather than within a single synovial cavity is why the swelling is fusiform and continuous rather than localised.
Alternative names: Dactylitis, sausage finger, sausage toe
Other associated named signs: Ivory phalanx, pencil-in-cup deformity, fluffy periostitis, and spina ventosa of tuberculous dactylitis
References
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